The disclosed invention relates in general to a light coupler and relates more particularly to a light coupler for use in optical reflectometry on a monomode glass fiber. In optical reflectometry, a light beam is injected into one end of an optical fiber and then reflections of the light back to that end of the fiber are used to locate the spatial locations of cuts, fractures or other faults in the optical fiber. Typically, the light is injected from a pulsed laser and the light reflected back by the fiber faults coupled out to a measuring output by a beam splitter contained in the coupler. A measuring instrument is coupled to the output to measure the intensity of reflected light as a function of time.
The design of the coupler depends largely on whether a single mode fiber or a multimode fiber is being tested. Single mode fibers typically have a diameter on the order of the wavelength of light (9-12 microns diameter) so that only one specific mode of light can propagate along the fiber. Multimode fibers are typically thicker (50-120 microns diameter) than single mode fibers so that many modes can propagate down the fiber. Multimode fibers typically exhibit a larger attenuation than single mode fibers.
In a multimode fiber, if a linearly polarized light beam is injected into the fiber, the difference in propagation velocity of the different modes results in a depolarization of the light beam within about ten centimeters of the end of the injection end of the fiber. In contrast to this, in a single mode fiber the linear polarization is retained over large distances on the order of a couple kilometers. In addition, the light reflected back to the injection end of the fiber is also linearly polarized. However, thermal and mechanical stresses in a single mode fiber can rotate the direction of polarization of the light. If a polarized beam splitter is used to couple the light out of the fiber, then such rotations can lead to spurious effects on the amount of reflected light coupled out of the fiber. Therefore, a nonpolarized beam splitter is used to couple light into and out of the fiber so that the amount of light coupled out is substantially independent of the polarization of the reflected light. Such a nonpolarized beam splitter can be produced from semi-transparent mirrors or fiber optic beam splitters that have a splitting ratio that is substantially independent of the polarization of the reflected light. Unfortunately, such a coupler also reduces by 50% the amount of light coupled into the fiber and then further reduces by 50% the amount of light coupled out of the fiber to the measuring device.